Wireless communication using radios can be used for communications on land, in the air, at sea, or on opposite sides of the world. Communication from point to point on the ground is commonly accomplished with antennas such as monopoles or dipoles. A dipole, for example, has two elements approximately a quarter wave in length, arranged in a shared axial alignment configuration with a small gap between the two elements. Each element of the dipole can be fed with a current 180 degrees out of phase from the other element. A monopole has one element approximately a quarter wave in length, and operates in conjunction with a ground plane, which mimics the missing second element.
Monopoles and dipoles are generally used for line-of-sight (LOS) communications. Obstructions such as mountains, or long distances, relative to the curve of the earth's surface between the transmitter and receiver, can prevent the reception of LOS electromagnetic signals. The relative positions and heights of the transmitter and receiver, as well as the power output of the transmitter and sensitivity of the receiver determine the total successful communication distance for LOS.
To overcome LOS communication distance limitations, satellite communications (SATCOM) has been developed. Orbiting satellites have transceivers that can relay communications back and forth from the earth's surface or to other satellites, allowing communication virtually anywhere in the world.